Eddy current probes are used to scan the surface of a material to locate flaws. A problem arises in distinguishing flaws from mere irregularities in the shape of the surface. For example, an irregularity may appear as a small bump which results from the grinding of a welding bead of a seam, yet may not be considered a flaw. However, the bump tends to mask eddy current signals produced by flaws near it.
Further, if the eddy current probe is scanned along the surface up to the bump and then either accidentally bounced away from the surface because of impact with the bump or intentionally raised to hurdle the bump, the bouncing and raising themselves produce lift-off signals which tend to mask the signals produced by the flaws.
Similarly, lift-off signals occur when scanning is undertaken of a fillet on a boss of a gas turbine engine rotor when the eddy current probe reaches a surface irregularity caused by, not a welding bead, but a bump left by the parting line of a mold used to shape the boss in an electrochemical machining process.